In a voltage conversion circuit in a DC-DC convertor and the like, for example, a direct current reactor has heretofore been used as an inductance part.
The direct current reactor has a magnetic core (core) that is made of a soft magnetic material and the like and may be varied in shape and a winding area that is wound around the magnetic core. A current that changes cyclically is ordinarily applied to the direct current reactor in a state where a direct current is biased.
The direct current reactor of the above-described type is required to have a constant inductance in a relatively wide operation electric current range. When the inductance is fluctuated, for example, a trouble such as a fluctuation in direct current voltage to be outputted occurs.
For the purpose of satisfying the above-described requirement, a gap has heretofore been formed in the magnetic core of the direct current reactor. By the formation of the gap in the magnetic core, a magnetic resistance of the magnetic core is increased to suppress magnetic saturation, thereby improving direct current superimposition characteristics of the reactor.
Also, in the gap, an insulation material such as glass epoxy material or the like is ordinarily used as a gap material, and a permanent magnet or the like may also be provided in some cases.
For instance, JP-A-2003-109832 discloses a magnetic core and an inductance part, wherein a bond magnet formed of a rare earth sintered magnet powder (coercive force: 3979 kA/m=50 kOe or more) and a resin is inserted into a gap formed on a magnetic path of the magnetic core.
Also, JP-A-50-133453 discloses an inductance element (reactor) that applies a magnetic bias by a permanent magnet that is inserted in a clearance of a magnet.
Also, JP-A-2007-123596 discloses a direct current reactor of a magnet bias type, wherein a permanent magnet is disposed so as to generate a bias magnetic field, whereby a magnetic flux formed by a coil and a magnetic flux formed by the permanent magnet cancel each other out.
However, the conventional techniques have the following problems.
In the case where the permanent magnet is disposed in the gap of the magnetic core in the direct current reactor, the direct current superimposition characteristics are improved. Such an improvement is achieved since the magnetic saturation of the magnetic core is alleviated by the bias magnetic field generated by the magnet.
However, such an effect is exhibited only when the magnetic force of magnet that decides the size of the bias magnetic field is stabilized in a use temperature range of the reactor.
Although the above-described effect is expected by the direct current reactor in which the permanent magnet is disposed in the gap of the magnetic core, a product has not yet been provided in actuality as a reactor to which a high electric current is applied. Therefore, under a current situation, the direct current reactor in which the gap material such as glass epoxy resin is disposed in the gap of the magnetic core is the mainstream product.
Reasons for the above-described current situation include disappearance of the magnet-based bias effect due to irreversible demagnetization of the permanent magnet by heat caused in a temperature range (for example, from about −40° C. to about 150° C.) at which the direct current reactor is usually used and the like.
As disclosed in JP-A-2003-109832, it is considered that the above problem may be solved by using a sintered magnet powder having a remarkably large coercive force (about 3979 kA/m).
However, a relationship between a coercive force (iHc) and a residual magnetic flux density (Br) of rare earth magnet is so-called a trade-off relationship in which one of them is reduced when the other one is increased.
Accordingly, in the case that the above-described large coercive force is set to about 3979 kA/m, it is difficult to keep the residual magnetic flux density to 0.25 T or more, so that it is difficult to ensure a residual magnetic flux density required for generating a sufficient bias magnetic field. Therefore, it is considered that it is difficult to actually achieve improvement in direct current superimposition characteristics.
Consequently, it is considered to use a sintered magnet powder having a coercive force that is required from the practical point of view. However, according to the investigations made by the inventors, it was revealed that sufficient bias magnetic field is not generated and a problem of an increase in noise during use of the direct current reactor occurs with the use of such sintered magnet powder.
In JP-A-50-133453, demagnetization of the magnet at a temperature in actual use and in a diamagnetic field is not fully considered. Also, in JP-A-2007-123596, since it is difficult to effectively bias the magnetic flux of a magnet, a stronger magnet is required to thereby cause an increase in size of the reactor. Further, since it is difficult to generate the appropriate bias magnetic field, it is considered that it is impossible to achieve an effect of reducing noise.